A recent article by Samuel Arbesman in the science magazine Nautilus discusses the extraordinary sounding possibility that - just perhaps - a search for extraterrestrial intelligence could be made by looking at our DNA.
Yes, that's right, at DNA. Not just human DNA either, but the codes of all living organisms. It's a fascinating, if slightly fanciful idea that has come up a few times in recent years.
If a species wanted to communicate across space and time (lots of time), encoding a message in the fabric of life itself - in the base-pair sequences - might be a way to do it. And people have indeed looked.
But, as Arbesman points out, one of the more intractable problems with using DNA to leave a 'secret' message from the stars is that evolution is likely to ditch or scramble code that is not contributing to the positive function of living things. Thus, if a technologically advanced civilization left a molecular calling card a few million or billion years ago it wouldn't be present or recognizable now, unless it was constructed with enough ingenuity to be a vital part of the function of life. Similarly, if some bizarrely alien retrovirus showed up a billion years ago, we might be hard pushed to spot its signature in today's genetic code.
However, these ideas prompt me to revisit the question of how a suitably motivated ET might leave a message to mark its once-upon-a-time presence in our solar system. A wee bit speculative? You bet, but fun to consider nonetheless.
One of the many potential 'solutions' to the Fermi Paradox (if alien life is not very rare, why hasn't it shown up yet?) is that the timing is off. Biologically modern humans have only been around a couple hundred thousand years. That's a narrow window of opportunity for a tête-à-tête, even if pan-galactic species are indeed buzzing around the great emptiness of interstellar space.
If we presume (and it's quite a presumption) that ET really wanted to leave a calling card, in the off-chance that someone clever enough to read it might come along later, what would be the best way to do this?
Thinking Earth-centrically, our planet is probably not a great place to leave anything that needs to survive for millions to billions of years and be guaranteed of retrieval. Geophysical and geochemical cycles will wreak havoc with artificial structures, as will the attentions of microbial organisms - especially the hardiest that forage for delicious inorganic matter. And there is no point in leaving something that will be too difficult to find - buried within the rocky heart of ancient mountains for example. Yet you also want to make sure than when a message is discovered, it'll be discovered by a species clever enough to get the message.
Arthur C. Clarke, as happens shockingly often, had a prescient insight to this problem. In his 1948 story The Sentinel, Clarke describes an artifact left on the Moon, a warning beacon to tell a destructive species (us) that we should tread carefully as we reach out to the stars. And of course in his 2001, an alien monolith buried in the lunar soil is the lynchpin for humanity's sojourn into the cosmos.
If you buzzed through the solar system and thought that the Earth was the likeliest place for intelligent life to crop up, its moon is an excellent place to leave a 'call me later' card. The lunar environment is ancient, largely stable, and essentially free from the kind of chemical and biochemical erosion that exists on the Earth. Material does hit the surface, but big stuff is rare, so you don't need a huge amount of protection. A mound of soft lunar regolith will do the trick.
So where exactly would you put your message on the Moon?
You'd put somewhere interesting, a place that a space-faring species is going to want to visit. I can think of three obvious choices: polar water ice deposits, lava-tube caves, or one of the funkiest stand-out natural features - like the Aristarchus plateau.
In each case, these are places that an inquisitive (if not ambitious) species will eventually study.
But there is a flaw to this idea. It assumes that an ET surveying our solar system would consider Earth as the best bet for a receptive audience. Depending on when a visit occurs, Mars might seem the most appealing, or one of the icy moons and their subsurface oceans (which are likely to be long-lived in many instances).
For Mars, placing an artifact on one of the martian moons - Phobos or Deimos - would seem logical for many of the same reasons that Earth's moon is a good spot. The disadvantage of Phobos is that it will likely be tidally disrupted in about 50 million years from now. A clever ET might have surmised this eventual fate and decided on the smaller, outer, moon Deimos instead.
For somewhere like Europa, options are a bit more limited. This is tricky terrain, both on the surface - which is relatively youthful and likely active on geological timescales - and in the Jovian orbital space, where particle radiation is intense.
Having said that, plopping your 'message' inside Europa might be a good idea. Either an ambitious species from elsewhere in the solar system will eventually want to drill into the moon, or if Europa's ocean eventually gave rise to sentient life (admittedly a stretch given the low chemical energy likely available), it would come across as the equivalent of a message floating in a bottle. The catch would be how to get the timing right for discovery. I don't know how you'd do this.
There's another neat option to ensure that whoever gets the message has reached a sufficiently advanced technological level to make sense of it. That's to place it at the 'gateway' to the rest of the universe. The outer solar system.
Assuming that this is the region aligned with the planetary orbits (for reasons of minimizing energy expenditure for any vehicle hell bent on getting out), that could mean Neptune. It could also mean one of the large Kuiper-belt objects that a species might visit either out of curiosity, or conceivably to use as a staging or refueling post. One of those objects is Pluto.
I'm not suggesting with any great seriousness that when New Horizons zips through the Plutonian system in the summer of 2015 it might spot a message from the last alien visitors. But we still have no real answer to the Fermi Paradox, and in that sense the possibilities are wide open. It might be worth just a teeny moment of contemplation as the first close-up data from Pluto returns across four and a half light hours. You never know.
[Minor disclosure: I am on the advisory board for the magazine Nautilus, and sometimes write for them, but that is coincidental to my referencing the Arbesman article].